His versatility amazes other runners, including Jurek, who today is a friend. Jornet has been able to run the very short mountain races like a vertical kilometer race that's over in a couple of hours, Jurek says -- and then, he adds, Jornet can turn around and win the Western States 100-Mile Endurance Run in California's Sierra mountains, arguably the world's most prestigious ultrarun. (Jurek himself won the Western States seven consecutive times between 1999 and 2005.) It's a little like an Olympic-champion sprinter winning the Boston Marathon.

How is Jornet able to do these things? In part because of his upbringing in the mountains:

Even among top athletes, Jornet is an outlier. Take his VO2 max, a measure of a person's ability to consume oxygen and a factor in determining aerobic endurance. An average male's VO2 max is 45 to 55 ml/kg/min. A college-level 10,000-meter runner's max is typically 60 to 70. Jornet's VO2 max is 89.5 -- one of the highest recorded, according to Daniel Brotons Cuixart, a sports specialist at the University of Barcelona who tested Jornet last fall. Jornet simply has more men in the engine room, shoveling coal. "I've not seen any athletes higher than the low 80s, and we've tested some elite athletes," says Edward Coyle, director of the Human Performance Laboratory at the University of Texas at Austin, who has studied the limits of human exercise performance for three decades.

He also seems at one with nature and the mountains more than most people:

Observers and competitors describe him as someone who draws endurance and vitality, Samson-like, from being among high peaks. Runners who have served as pacesetters for him have told me with amazement how, when he was midrace at Lake Tahoe, Jornet didn't run with his head down in focused misery but instead brushed the hairgrass and corn lily that grew along the trail with his fingertips and brought the smell to his nose, as if he were feeding off the scenery. Sometimes in his all-day solitary runs, stopping only to eat berries, he can seem half-feral, more mountain goat than human. He likes to move fast and touch rock and feel wild, he told me; he feels most at ease and performs best when wrapped by the silence and beauty of the mountains. He can't abide cities for more than a few hours. The sea -- its unrelenting horizontality -- scares him. Leading long races like Western States, he's been known to stop and exclaim at a sunrise, or wait for friends to catch up so he can enjoy the mountains with them instead of furthering his lead. "It's almost insulting," Krupicka told me. But it's just Kilian being Kilian, Krupicka said. "He's not rubbing it in anyone's face. He's truly enjoying being out there in the mountains, and he's expressing that."

Update: In the New Yorker, Stephen Kurczy has an update on Jornet and his quest to establish a "fastest known time" on some of the world's tallest mountains. He's already bagged Mont Blanc, Matterhorn, and Denali with an Everest attempt forthcoming.

Perhaps the most valuable thing Jornet gained on Aconcagua was a lesson about altitude. This was the highest mountain he had ever raced, and it will inform the next leg of his project, a trip to Everest this spring, and force him to invest more time in acclimatization -- a challenge for someone more accustomed to just going. Montaz-Rosset told me that the aim will be to run up and down the northern Tibet side starting from one of the last inhabited places before base camp, Rongbuk Monastery, at sixteen thousand four hundred feet. That route would translate to some twelve thousand five hundred feet of elevation gain -- a little less than on Aconcagua. Jornet intends to carry only a backpack, without oxygen or the assistance of fixed ropes or other climbers.